Monthly Archives: October 2015

Understanding blobs of spacetime

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Read the full article for free* in Classical and Quantum Gravity:
Spacetime condensation in (2+1)-dimensional CDT from a Hořava-Lifshitz minisuperspace model
Dario Benedetti and Joe Henson 2015 Class. Quantum Grav. 32 215007
arXiv:1410.0845
*until 25/11/15

Can we explain the condensation of spacetime seen in numerical simulations of Causal Dynamical Triangulations?

Dario Benedetti

Dario Benedetti is a CNRS researcher at the Laboratoire de Physique Theorique at Orsay, France. His work focuses on various approaches to quantum gravity including CDT and asymptotic safety.

In the search of a quantum theory of gravity, it is not often that we are faced with the challenge of explaining some novel physical phenomenon: experiments are notoriously lacking, and theoretical questions usually involve clarifying the features of the different approaches, or the paradoxes of established theories. One of the most exciting aspects of Causal Dynamical Triangulations (CDT) is that numerical studies can produce unexpected results, which must then be explained, much like in mainstream statistical mechanics research.

Our paper, published in Classical and Quantum Gravity, is concerned with providing such an explanation Continue reading →

The gravitational Hamiltonian, first order action, Poincaré charges and surface terms

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Read the full article for free* in Classical and Quantum Gravity:
The gravitational Hamiltonian, first order action, Poincaré charges and surface terms
Alejandro Corichi and Juan D Reyes 2015 Class. Quantum Grav. 32 195024
arXiv:1505.01518
*until 18/11/15

Ever since Einstein and Hilbert were racing to complete the general theory of relativity, almost 100 years ago, having a variational principle for it was at the forefront of the theoretical efforts. An action and the variational principle accompanying it are the preferred ways to describe a physical theory. At the classical level, all the information one can possibly ask about a physical system is conveniently codified into a single scalar function S. Additionally, in covariant approaches to quantum mechanics, the action S provides, through the path integral, a fundamental link between the classical and quantum descriptions. Ideally, the Hamiltonian structure of the theory itself -the starting point for canonical quantization- may too be extracted from the same action. Continue reading →

CQG Letters – open for submission

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Clifford Will

Clifford Will is the Editor-in-Chief of Classical and Quantum Gravity

I am very pleased to announce the launch of CQG’s new Letters section.

In a recent survey of CQG’s authors and readers, we identified a strong desire among gravitational physicists for CQG to launch this section.

The Letter article type is now available and I invite you to submit your next Letter to the journal.

The benefits

CQG’s peer review will ensure the best possible consideration for your Letter – certifying, not just Continue reading →

Gravity and scalar fields: live long and prosper?

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Read the full article for free* in Classical and Quantum Gravity:
Tensor-multi-scalar theories: relativistic stars and 3 + 1 decomposition
Michael Horbatsch, Hector O Silva, Davide Gerosa, Paolo Pani, Emanuele Berti, Leonardo Gualtieri and Ulrich Sperhake 2015 Class. Quantum Grav. 32 204001
arXiv:1505.07462
*until 16/12/15

Ulrich Sperhake et al

Large panel: Ulrich Sperhake and Emanuele Berti under Isaac Newton’s famous apple tree at Woolsthorpe Manor that (allegedly) started it all.
Clockwise in the small panels: Davide Gerosa, Hector O. Silva, Paolo Pani, Leonardo Gualtieri and Michael Horbatsch.

Newton’s theory of gravity was a spectacular achievement: for about two centuries, a simple law based on empirical observation was a perfect explanation for the behavior of gravity throughout the Solar System. Some cracks in this perfect edifice emerged around 1840, when François Arago, the director of the Paris Observatory, suggested to the French mathematician Urbain Le Verrier to study the details of Mercury’s orbital motion around the Sun using Newton’s gravity. Predictions from Le Verrier’s theory famously failed to match the observations. Mercury’s perihelion advances each time it orbits around the Sun. Most of the perihelion precession could be explained as due to the Continue reading →

4 questions about CQG Letters

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Adam Day

Adam Day is the Publisher of Classical and Quantum Gravity and CQG+

At the 2015 CQG Editorial Board meeting in London, it was agreed that a subset of the gravitational physics community should be surveyed to find out if there was interest in launching a Letters section for CQG.

In this survey, a number of gravitational physicists were asked to answer a set of questions about Letters. Just to make things interesting, invitees were split into 2 groups: one group was asked to think as authors and the other as readers.

Here are the results. Continue reading →

The curvature on a black hole boundary

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Read the full article for free* in Classical and Quantum Gravity:
On the Bartnik mass of apparent horizons
Christos Mantoulidis and Richard Schoen 2015 Class. Quantum Grav. 32 205002
arXiv:1412.0382
*until 04/11/15

Christos Mantoulidis

Christos Mantoulidis is a graduate student in Mathematics at Stanford University.

In our latest CQG paper we study the geometry (i.e. curvature) of apparent horizons and its relationship with ADM mass.

We were motivated by the following two foundational results in the theory of black holes in asymptotically flat initial data sets (slices of spacetime) satisfying the dominant energy condition (DEC):

  1. Apparent horizons are topologically equivalent to (one or more) two-dimensional spheres.(1)
  2. When the initial data set is additionally time symmetric (totally geodesic in spacetime), the apparent horizon’s total area A is bounded from above by the slice’s ADM mass per A \leq 16\pi m^2. This is called the Penrose inequality.(2) Equality is only achieved on Schwarzschild data, whose apparent horizon is a single sphere with constant Gauss curvature.

One then naturally wonders: Continue reading →